# std::packaged_task

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Defined in header `<future>` |  |
template< class > class packaged_task; //not defined | (1) | (since C++11)
template< class R, class ...ArgTypes > class packaged_task<R(ArgTypes...)>; | (2) | (since C++11)

The class template `std::packaged_task` wraps any Callable target (function, lambda expression, bind expression, or another function object) so that it can be invoked asynchronously. Its return value or exception thrown is stored in a shared state which can be accessed through std::future objects.

Just like std::function, `std::packaged_task` is a polymorphic, allocator-aware container: the stored callable target may be allocated on heap or with a provided allocator. | (until C++17)

### Member functions

(constructor) | constructs the task object (public member function)
(destructor) | destructs the task object (public member function)
operator= | moves the task object (public member function)
valid | checks if the task object has a valid function (public member function)
swap | swaps two task objects (public member function)
##### Getting the result
get_future | returns a std::future associated with the promised result (public member function)
##### Execution
operator() | executes the function (public member function)
make_ready_at_thread_exit | executes the function ensuring that the result is ready only once the current thread exits (public member function)
reset | resets the state abandoning any stored results of previous executions (public member function)

### Non-member functions

std::swap(std::packaged_task) (C++11) | specializes the std::swap algorithm (function template)

### Helper classes

std::uses_allocator<std::packaged_task> (C++11) (until C++17) | specializes the std::uses_allocator type trait (class template specialization)

### Deduction guides(since C++17)

### Example

Run this code

    #include <cmath>
    #include <functional>
    #include <future>
    #include <iostream>
    #include <thread>

    // unique function to avoid disambiguating the std::pow overload set
    int f(int x, int y) { return std::pow(x, y); }

    void task_lambda()
    {
        std::packaged_task<int(int, int)> task([](int a, int b)
        {
            return std::pow(a, b);
        });
        std::future<int> result = task.get_future();

        task(2, 9);

        std::cout << "task_lambda:\t" << result.get() << '\n';
    }

    void task_bind()
    {
        std::packaged_task<int()> task(std::bind(f, 2, 11));
        std::future<int> result = task.get_future();

        task();

        std::cout << "task_bind:\t" << result.get() << '\n';
    }

    void task_thread()
    {
        std::packaged_task<int(int, int)> task(f);
        std::future<int> result = task.get_future();

        std::thread task_td(std::move(task), 2, 10);
        task_td.join();

        std::cout << "task_thread:\t" << result.get() << '\n';
    }

    int main()
    {
        task_lambda();
        task_bind();
        task_thread();
    }

Output:

    task_lambda: 512
    task_bind:   2048
    task_thread: 1024

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3117 | C++17 | deduction guides for `packaged_task` were missing | added

### See also

future (C++11) | waits for a value that is set asynchronously (class template)

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